Clinical disease due to cytomegalovirus (CMV) is recognized in up to forty percent of patients with advanced HIV disease. Retinitis is the most common clinical manifestation of CMV, and develops in approximately forty-two percent of individuals with fewer than 50 CD4 cells/mm3 within twney-seven months. It is known that CMV retinitis in AIDS results from reactivation of a previously acquired latent CMV infection. Nearly all patients who are HIV infected have serological evidence of CMV infection, which has been acquired earlier in life through mucosal exposure. The purpose of this study is to measure the levels of cytomegalovirus (CMV) in the blood of persons with HIV who are being screened for CMV retinitis, or who have been diagnosed with and are receiving treatment for CMV retinitis.